The leaked NSA report has raised fresh concerns over Moscow's meddling in the 2016 US presidential polls.

A top-secret report by National Security Agency (NSA), leaked on Monday, suggested that Russian hackers attacked at least one US voting software supplier days before 2016 presidential election.

The report has raised fresh concerns over Moscow's meddling in the 2016 US presidential polls.

Immediately after the NSA report was published online by The Intercept, a US federal agency contractor Reality Leigh Winner was arrested by the FBI for allegedly leaking the classified report.

"Exceptional law enforcement efforts allowed us quickly to identify and arrest the defendant," said Deputy Attorney General Rod J Rosenstein.

"Releasing classified material without authorisation threatens our nation's security and undermines public faith in government. People who are trusted with classified information and pledge to protect it must be held accountable when they violate that obligation," Rosenstein said.

According to the criminal complaint, 25-year-old Winner is a contractor with Pluribus International Corporation assigned to a US government agency facility in Georgia.

The classified report of the NSA has alleged that the Russian military intelligence launched a 2016 cyberattack on a voting software company.

Winner has been employed at the facility since on or about February 13, and has held a top-secret clearance during that time.

"On May 9, Winner printed and improperly removed classified intelligence reporting, which contained classified national defense information from an intelligence community agency, and unlawfully retained it. Approximately a few days later, Winner unlawfully transmitted by mail the intelligence reporting to an online news outlet," it said.

According to the complaint, Winner agreed to talk with agents during the execution of the warrant.

During the conversation, Winner admitted intentionally identifying and printing the classified intelligence reporting at issue despite not having a "need to know," and with knowledge that the intelligence reporting was classified.

Winner further admitted removing the classified intelligence reporting from her office space, retaining it, and mailing it from Augusta to the news outlet, which she knew was not authorised to receive or possess the documents, the complaint said.

According to the document published by The Intercept, the Russian Military Intelligence executed a cyberattack on at least one US voting software supplier and sent spear-phishing emails to more than 100 local election officials just days before last November's presidential election.

The report, dated May 5, 2017, isthe most detailed US government account of Russian interference in the election that has yet come to light, it said.

"While the document providesa rare windowinto the NSA'sunderstanding of the mechanics of Russian hacking, it does not show the underlying "raw" intelligence on which the analysis is based.A US intelligence officer who declined to be identified cautioned against drawing too big a conclusion from the documentbecause a single analysis is not necessarily definitive," The Intercept said.

Meanwhile, WikiLeaks founder Julian Assange said that Winner must be supported.

"She is a young woman accused of courage in trying to help us know," Assange said.

"It doesn't matter why she did it or the quality [of] the report," Assange added in a separate tweet.

"Acts of non-elite sources communicating knowledge should be strongly encouraged," he said.

According to The Intercept, the report indicates that Russian hacking may have penetrated further into US voting systems than was previously understood.

"Itstates unequivocally in its summary statement that it was Russian military intelligence, specifically the Russian General Staff Main Intelligence Directorate, or GRU, that conductedthe cyberattacks described in the document," it said.